Tunnel freezers are well known in the art. The conventional tunnel freezer comprises:
(a) an elongated tunnel having a first end and a second end; PA1 (b) an item entrance port located at or near the first end for introducing items to be frozen into the tunnel; PA1 (c) an item exit port located at or near the second end for withdrawing the frozen items from the tunnel; PA1 (d) a refrigerant admission port located at or near either end for introducing a refrigerant into the tunnel; PA1 (e) a refrigerant discharge port located at or near that end of the tunnel which is opposite from the refrigerant admission port for withdrawing the refrigerant from the tunnel; and PA1 (f) a conveyor belt for moving the items from the item entrance port, through the tunnel, and to the item exit port.
See for example U.S. Pat. No. 4,800,728 by Klee.
Tunnel freezers which incorporate an initial immersion section are also known in the art. See for example the article by Robert Sperber in the January 1990 issue of Food Processing entitled "Hybrid freezing system doubles capacity". Such freezers take advantage of the excellent conduction heat transfer that occurs when the conveyor belt is initially immersed in a bath of liquid refrigerant. As noted in this article, the remainder of the "hybrid tunnel freezer" which is downstream of the initial immersion section is based on conventional food freezing technology, namely passive convection heat transfer.